fail gracefully if slices-dir is incorrect

[shadielfares]

kubz run --slices-dir ~/Downloads/kubz_models/s3 --metadata-path ~/Downloads/kubz_models/s3/metadata.json --output-file ~/Downloads/kubz_models/s3/output/prove.json       



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Distributed zkML Toolkit

Running inference...
Traceback (most recent call last):
  File "/Users/shadielfares/Downloads/kubz/bin/kubz", line 33, in <module>
    sys.exit(load_entry_point('kubz', 'console_scripts', 'kubz')())
  File "/Users/shadielfares/Downloads/kubz/main.py", line 74, in main
    run_inference(args)
  File "/Users/shadielfares/Downloads/kubz/src/cli/run.py", line 126, in run_inference
    default_input_file = os.path.join(args.model_dir, "input.json")
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/posixpath.py", line 76, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType```